Incompatibility with const fields in mutable struct

[Tortar]

Currently

@proto mutable struct B
          const x::Int
       end

throws

ERROR: LoadError: BoundsError: attempt to access 1-element Vector{Any} at index [2]
Stacktrace:
 [1] getindex
   @ ./essentials.jl:13 [inlined]
 [2] #5
   @ ~/.julia/packages/ProtoStructs/4sIVY/src/ProtoStruct.jl:47 [inlined]
 [3] iterate
   @ ./generator.jl:47 [inlined]
 [4] _collect(c::Vector{Expr}, itr::Base.Generator{Vector{Expr}, ProtoStructs.var"#5#12"}, #unused#::Base.EltypeUnknown,
 isz::Base.HasShape{1})
   @ Base ./array.jl:802
 [5] collect_similar(cont::Vector{Expr}, itr::Base.Generator{Vector{Expr}, ProtoStructs.var"#5#12"})
   @ Base ./array.jl:711
 [6] map(f::Function, A::Vector{Expr})
   @ Base ./abstractarray.jl:3263
 [7] var"@proto"(__source__::LineNumberNode, __module__::Module, expr::Any)
   @ ProtoStructs ~/.julia/packages/ProtoStructs/4sIVY/src/ProtoStruct.jl:41
in expression starting at REPL[62]:1

Reiterate the same thing I said in #15 :D